1. Field of the Invention
The invention relates to a position detector on which a user indicates positions with a pen-shaped position indicator, and a display device having the position detector, and particularly to a position detector for detecting positions utilizing an electromagnetic effect.
2. Description of the Related Art
There are various kinds of input devices, such as keyboards, mice, and joysticks, utilized with electronic apparatuses such as personal computers (PCs) and PDAs (Personal Digital Assistants). Among these input devices, there is a so-called pen tablet that allows a user to input coordinates by drawing images or graphics on a flat-board device using a pen-shaped input device.
FIG. 1 is a perspective view illustrating a configuration example of a pen tablet. The pen tablet shown in FIG. 1 includes a position indicator (pen) 10 having a pencil shape, and a sensor (tablet) 20 including a sensor substrate 27 and a drawing region 26-a. The sensor substrate 27, which will be described in detail later, includes coil(s) for detecting positions; that is, the induced voltage generated in the coil(s) is used to detect coordinates of positions indicated by the position indicator 10 in the drawing region 26-a of the sensor 20.
The pen tablet of this kind includes a tablet PC which includes the sensor 20 incorporated in a display of a computer (particularly, a note-type personal computer). A user inputs coordinates of arbitrary positions by directly drawing on the surface of the display with the position indicator.
First, the principle of such a pen tablet will be described. It is noted that the word “pen tablet” is used synonymously with the word “position detector” in the present description.
The position indicator 10 and the sensor 20 in FIG. 1 each include coil(s). Operation of the position indicator 10 and the sensor 20 will later be described in detail; however, overviews thereof are briefly described as follows. An electromagnetic wave is first transmitted from the sensor 20 side coil(s) in a short period of time. The position indicator 10 side coil receives the electromagnetic wave and resonates at the substantially same frequency as that of the received electromagnetic wave. This means that energy is stored in the position indicator 10 side resonant circuit. The electromagnetic wave transmitted from the sensor side 20 coil(s) subsequently stops, and the energy stored in the resonant circuit is transmitted from the position indicator 10 side coil as an electromagnetic wave.
The electromagnetic wave transmitted from the position indicator 10 side coil is then received by the sensor 20 side coil(s), so that the current coordinates indicated by the position indicator 10 are determined.
Below describes in detail how coordinates indicated by the position indicator 10 are detected. FIG. 2 is a conceptual view illustrating a position detector (i.e., a pen tablet) having one sensor coil 21 in the sensor 20. The “coil” provided at the sensor 20 side is referred to as a “sensor coil”. The position indicator 10 includes a resonant circuit 13 having a coil 11 and a capacitor 12. The “coil” 11 is hereafter referred to as a “pen coil”.
The sensor coil 21 is provided at the sensor 20 side, and is connected to a transmitting-receiving changeover switch 24. The changeover switch 24 is connectable to a current driver 23 and to an amplifier 25, and switches between the current driver 23 and the amplifier 25, both of which are connected to the sensor coil 21.
Next, FIG. 2 illustrates operational steps of the position detector having such a configuration.
(1) First, the transmitting-receiving changeover switch 24 is connected to the current driver 23 for a certain amount of time (T1), and supplies an alternating current signal to the sensor coil 21 to generate an electromagnetic wave.
(2) The electromagnetic wave output from the sensor coil 21 is received by the pen coil 11, causing the resonant circuit 13 of the position indicator 10 to resonate.
(3) After the certain amount of time (T1) has elapsed, the transmitting-receiving changeover switch 24 switches to the amplifier 25 for a certain amount of time (T2).
(4) Then, during T2, no electromagnetic wave is supplied to the position indicator 10, and energy stored in the resonant circuit 13 causes the pen coil 11 to transmit an electromagnetic wave. While the electromagnetic wave is being transmitted for the certain amount of time (T2), no energy is supplied from outside to the resonant circuit 13, such that the amplitude of the electromagnetic wave transmitted is gradually attenuated, as shown in the reception current waveform in FIG. 2.
(5) The transmitting-receiving changeover switch 24 switches to the current driver 23 again for the certain amount of time (T1), and the same operation described in (1) is carried out.
The electromagnetic wave is transmitted and received in this manner between the sensor 20 side coil and the position indicator 10 side coil. In a case where a plurality of sensor coils 21 are arranged in the sensor 20, the coordinate indicated by the position indicator 10 is determined by detecting which one of the sensor coils the position indicator side coil is communicating with.
FIG. 3 is a view conceptually illustrating the distribution of induced voltage generated by the position indicator 10 in the sensor 20. The position indicator 10 includes the resonant circuit 13 having the coil 11 and the capacitor 12. The sensor 20 includes a plurality of sensor coils 21, which are illustrated as four sensor coils 211 to 214 in the example of FIG. 3.
The sensor coils 211 to 214 are each connectable to a sensor coil changeover switch 22, with which the sensor coils are individually operated. The sensor coil changeover switch 22 is connected to the transmitting-receiving changeover switch 24, so that the sensor coil 21 is switched either to transmit or to receive an electromagnetic wave. The transmitting-receiving changeover switch 24 is also connectable to the current driver 23 and to the amplifier 25. The current driver 23 drives an alternating current signal.
Below describes how a coordinate indicated by the position indicator 10 is detected by the plurality of the sensor coils 21 disposed in the sensor 20.
(a) First, the sensor coil changeover switch 22 is connected to the sensor coil 211, and the transmitting-receiving changeover switch 24 is connected to the current driver 23. The sensor coil 211 transmits an electromagnetic wave in this manner.
(b) Next, the electromagnetic wave is transmitted and received between the position indicator 10 and the sensor 20 via the transmitting-receiving changeover switch 24, as described above in steps (1) through (5), and hence the value of the induced voltage of the sensor coil 211 is detected.
(c) The sensor coil changeover switch 22 sequentially switches between the sensor coils 212, 213, 214 to operate the aforementioned steps (1) through (5).
The sensor coil changeover switch 22 sequentially switches between the sensor coils 211, 212, 213, 214 to detect the magnitude of the induced voltage for each of the respective sensor coils 211, 212, 213, 214. Thus, the intensity distribution of electromagnetic wave output from the position indicator 10 is obtained for each time the position indicator 10 is placed on or near the sensor coils 211, 212, 213, or 214, as shown in the graph of FIG. 3. The positions of the sensor coils 211 to 214 are represented by points (X1 to X4) plotted on an X-axis of the graph.
The sensor coils 21 each have a long and thin shape, some of which are arranged in a longitudinal axis (Y-axis) direction (not shown), and some of which are arranged in a short axis (X-axis) direction. In FIG. 3, the sensor coils 21 are arranged in the short axis direction; that is, the sensor coils 21 are arranged along the X-axis direction. The X coordinates on the graph correspond to the central points of the short axes of the sensor coils 21. The X coordinates are optionally determined either as the central points of the sensor coils 21 as described, or as other points of the sensor coils 21.
The induced voltage V1 to V4 detected at the respective sensor coils 211 to 214 are plotted along the X-axis at X-coordinates X1 to X4 to approximate a curve, to thereby produce the graph in FIG. 3. In this graph, the peak of the curve corresponds to a coordinate Xc representing the central axis of the pen coil 11. Thus, X coordinates indicated by the position indicator 10 can be calculated. Likewise, other sensor coils may be arranged to intersect with these sensor coils 21 at right angles, so that Y coordinates indicated by the position indicator 10 can also be calculated.
Even when the position indicator 10 is moved at an interval shorter than the interval between the sensor coils 21, the distance between the pen coil 11 and the adjacent sensor coils 21 will still vary, thereby slightly changing the induced voltage generated in each of the sensor coils 21. Thus, the coordinates indicated by the position indicator 10 can be calculated based on this change.
In this case, the resolution of the coordinates that can be detected is higher than the interval between the sensor coils 21. To this end, it is preferable that the sensor coils 21 detect the induced voltage as precise as possible. The more precisely that coordinates can be indicated by the position indicator 10, and hence detected, the higher resolution the position detector can be.
FIG. 4 is a schematic view illustrating a structure of a typical position detector (i.e., a pen tablet). Since the position detector typically needs to detect X coordinates and Y coordinates, the sensor coils X (21X) and the sensor coils Y (21Y) are respectively arranged corresponding to the X-axis and Y-axis. The transmitting-receiving changeover switch 24, current driver 23, and amplifier 25, shown in FIG. 3, are arranged inside a coordinate calculating circuit 23.
The sensor coils X and the sensor coils Y are each configured to perform the aforementioned operations (a) through (c), so that the position detector obtains X coordinates and Y coordinates of the position indicator 10.
The user can input information with the position indicator 10 in a manner similar to drawing characters and pictures on paper with a pen. It is desirable to be able to change thickness of lines being drawn, by adjusting pen pressure or angles of the position indicator 10, such as by drawing lines with the position indicator inclined. Thus, the pen pressure or angles of inclination of the position indicator 10 need to be detected to achieve the change in drawing lines.
FIG. 5 illustrates one example of the position indicator 10 capable of performing such operation. As illustrated in the figure, the pen coil 11 surrounds the core 11b extending from a pen tip 14, and a capacitor 12 is engaged with one end of the core 11b. In the position indicator 10, when pressure is applied to the pen tip 14, the core 11b is moved to press the capacitor 12. The capacitor 12 usually has two electrodes facing each other, and a dielectric is located therebetween.
When a material used for the dielectric has the permittivity that varies with pressure, capacitance of the capacitor 12 can vary when force is applied to the pen tip 14. Thus, the resonance frequency of the resonant circuit 13 can vary based on the pen pressure.
The electromagnetic induction-type position detector transmits and receives an electric signal between the position indicator 10 and the sensor 20, utilizing electromagnetic induction. In other words, radio waves are transmitted and received between the position indicator 10 and the sensor 20. The electromagnetic wave is generally extremely weak, and tends to be adversely interfered with by an external noise. Further, the position detector should be able to accurately detect induced voltage generated by the pen coil 11 in order to achieve higher resolution, and to that end the sensor coils 21 should receive as little noise as possible.
Japanese Unexamined Patent Application Publication H07-115291 discloses a shield plate formed by arranging amorphous metal ribbons in parallel to form a ribbon plate, and stacking an aluminum plate on top of the resulting ribbon plate.